Series capacitor protective equipment can employ a nonlinear zinc oxide varistor to limit the magnitude of the voltage across the protected series capacitor. Under normal operating conditions load currents flow through the series capacitor such that the voltage across the capacitor is the product of the load current and the capacitive reactance. The voltage withstand of the capacitor is selected such that the capacitor voltage caused by the flow of load current is well within the voltage withstand capability of the capacitor. The varistor characteristic is selected such that under normal load current conditions the varistor current is limited to a few milliamperes. When a fault condition, for example a line to ground fault, occurs on the transmission line in which the series capacitor is connected the current through the capacitor increases. The current increase causes the capacitor voltage to increase and if the capacitor voltage is sufficiently high its voltage withstand capability is exceeded. To prevent the occurrence of excess voltage across the capacitor the zinc oxide varistor provides an alternative path for the fault current causing the excess capacitor voltage. However, the current flow through the zinc oxide varistor during line fault conditions may cause damage to the varistor if allowed to continue for prolonged periods of time. Because excessive energy is dissipated in the varistors some means must be provided therefore for limiting the total energy dissipation within the varistor itself.
One means commonly employed to protect equipment from excess energy dissipation is the employment of a parallel air gap to bypass at least a part of the energy developed during a fault situation. One of the problems involved with the employment of triggered air gap devices is to determine when the rate at which the energy is dissipated within the equipment becomes excessive. When the rate at which energy is dissipated in the equipment is too high the gap will not have sufficient time to operate before the equipment fails.
U.S. patent application 894,529, filed Apr. 7, 1978, discloses a circuit wherein both the quantity of energy dissipated within the protected varistor and the rate at which the energy is dissipated are determined. However, it has since been found that the rate of energy dissipation per se can determine when the varistor current must be bypassed to prevent varistor failure. This is particularly true when the fault occurs relatively close to the protected varistors and there is relatively little transmission line inductance to provide a current limiting impedance to the fault current. The rate of rise of energy is a direct function of the fault current so that the fault current may be used to determine when the varistor energy dissipation rate is excessive.